As a protection element that can prevent not only an overcurrent but also an overvoltage and is effectively used for a battery pack or the like for a portable electronic apparatus, a protection element has been proposed in which a low-melting-point metal body is placed on a charging/discharging electric current path of a battery cell so as to be interposed therebetween, with a heating resistor being disposed near the low-melting-point metal body. As shown in FIG. 9, a protection element 100 of this type generally has a structure in which a heating resistor 102 is formed on an insulating substrate 101 made of a ceramic material such as alumina, with a low-melting-point metal body 104 being disposed on the heating resistor 102.
The low-melting-point metal body 104 is formed on the insulating substrate 101, and connected to a conductive layer 106 connected between a battery cell and a charging/discharging circuit to form a charging/discharging electric current path of the battery cell. The conductive layer 106 is provided with a pair of surface electrodes 106a that are formed on the surface of the insulating substrate 101 so as to be opposed to each other, a pair of rear face electrodes 106b that are formed on the rear face of the insulating substrate 101 so as to be opposed to each other, and terminal electrodes 106c that connect the surface electrodes 106a and the rear face electrodes 106b respectively. One of the rear face electrodes 106b is connected to the battery cell, and the other thereof is connected to the charging/discharging circuit.
The heating resistor 102 is connected to a current control element through a conductive pattern, not shown, formed on the insulating substrate 101, and is also connected to the charging/discharging electric current path with the low-melting-point metal body 104 interposed therebetween. Moreover, the heating resistor 102 is formed on the surface of the insulating substrate 101, with a first insulating layer 107 made of a glass layer being interposed therebetween, and a second insulating layer 108 made of a glass layer is formed above the substrate. The low-melting-point metal body 104, which is formed on the second insulating layer 108 with the conductor 109 interposed therebetween, is also connected with the heating resistor 102.
Thus, in the case when upon occurrence of an abnormal state such as an overcharging state, an overdischarging state, or the like, of the battery cell, an excessive current flows through the heating resistor 102, the current is controlled by the current control element so as to flow from the battery cell to the heating resistor 102 so that the low-melting-point metal body 104 is fused by heat generated from the heating resistor 102, thereby shutting down the charging/discharging electric current path.